Hi Guys, I'm hoping you can help because I've exhausted my understanding of this !! I'm writing a decoder for messages that I receive on a socket. The Messages look like this : 
B2t^?\251 cEy]\213\247\3350002900262O0 NL000634286722^F^Q^S\210^F^S^S\
+210

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The rough guidance I have is that the format is :

Field Type Len

where "Alpha" is a string and Num is a binary field of type "uintx_t" (I guess uint8_t for example) I'm trying to unpack this and I can't seem to decode the MsgSeqNum and ClOrdID fields properly. I know the value of the messageType, ProtoOrVer and MsgLen fields because I can decode them successfully(verified with another program I have no source code access too)...however whatever I do I can't decode see to decode the MsgSeqNum and ClOrdID fields. I know from the output of the other program that the ClOrdID in this message is "4295075805" but no unpack strings I specify seem to work. If I use the following unpack statement just to get the field boundaries right : 
($messageType,$ProtoOrVer,$MsgLen,$MsgSeqNum,$ClOrdID,$OnBehalfOfCompI
+D,$rest) =unpack "A A n B32 B64 A*",$sourceMsg};

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then I get : 
08:02:43 : messageType == B
08:02:43 : ProtoOrVer == 2
08:02:43 : MsgLen == 116
08:02:43 : MsgSeqNum == 00000000000000001111111010010101
08:02:43 : ClOrdID == 000001001100011010100010100111101011101011010001
+1110010110111011
08:02:43 : OnBehalfOfCompID == 00029002
08:02:43 : rest == 62O0 NL000634286722^F^Q^S\210^F^S^S\210

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(This looks right because OnBehalfOfCompID is in the right place) Now, given that using "n" works for the MsgLen field(length of 2 from spec and n == 16 bits on pack perldoc page) I tried using n2 on the basis that the field is supposed to be twice as long but I get the following, which is obviously wrong : 
07:43:55 : messageType == B
07:43:55 : ProtoOrVer == 2
07:43:55 : MsgLen == 116
07:43:55 : MsgSeqNum == 0
07:43:55 : ClOrdID == 32681
07:43:55 : OnBehalfOfCompID == 8291
07:43:55 : rest == 17785

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On the basis that the field has a length of 4 x 8 == 32 I have also tried using combinations of "S", "L" and "N", for example : 
($messageType,$ProtoOrVer,$MsgLen,$MsgSeqNum,$ClOrdID,$OnBehalfOfCompI
+D,$rest) =unpack "A A n N N2 A*",$sourceMsg};

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to which I get : 
08:11:45 : messageType == B
08:11:45 : ProtoOrVer == 2
08:11:45 : MsgLen == 116
08:11:45 : MsgSeqNum == 32681
08:11:45 : ClOrdID == 543376761
08:11:45 : OnBehalfOfCompID == 1569433565
08:11:45 : rest == 00029002

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..which is also clearly wrong. I've had 4 hours hours sleep, I'm at my wit's end and I'm up against the wall to get this nailed today. Any help GREATLY appreciated !!

In reply to Lost in unpack hell by stevehicks

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